Ferrihollandite, BaMn⁴⁺₆Fe³⁺₂O₁₆, from Apuan Alps, Tuscany, Italy: description and crystal structure

The recent revisitation of the nomenclature of tunnel oxides belonging to the hollandite supergroup led to the definition of a new end-member, ferrihollandite, ideally BaMn⁴⁺₆Fe³⁺₂O₁₆. In particular, the type hollandite from Kajlidongri, Jhabua District, Madhya Pradesh, India, proved to be ferrihollandite. A new occurrence of ferrihollandite is here described from the manganese hydrothermal ores from Vagli, Apuan Alps, Tuscany, Italy, on the basis of chemical and X-ray diffraction data. Electron-microprobe data point to the following composition (in wt%): TiO₂ 3.06, MnO₂ 58.83, Mn₂O₃ 8.64, Fe₂O₃ 9.76, Al₂O₃ 0.79, ZnO 0.03, SrO 2.77, BaO 14.63, Na₂O 0.09, K₂O 0.05, sum 98.65. The structural formula, based on 16 oxygen atoms and 8 octahedral cations per formula unit, is (Ba_0.793Sr_0.222Na_0.024K_0.009)_∑=1.048(Mn⁴⁺_5.624Ti_0.318Fe³⁺_1.016Mn³⁺_0.910Al_0.129Zn_0.003)_∑=8.000O₁₆. The TG-DSC data show three main endothermic processes, related to the loss of water and the release of oxygen upon heating. The total mass loss is ~ 9.7 wt% in the temperature range between 25 and 1200 °C; the mass loss related to the release of water is ~ 1.5 wt%. Single-crystal X-ray diffraction study gives a monoclinic unit cell, a 10.0001(7), b 5.7465(4), c 9.8076(8) Å, β 90.713(2)°, V 563.56(2) ų, space group P2/n. The crystal structure has been solved and refined on the basis of 1675 observed reflections, with a final R₁ = 0.037. The basic structural features of ferrihollandite are the same as the 2.9 Å hollandite-type compounds, but with long-range ordering of the tunnel cations. The occurrence of superstructure reflections could be related to the ordered sequence Ba–H₂O–Ba–H₂O within the tunnels.